It quacks like a duck, sort of. But the mystery creature of the Antarctic is more likely a whale.

Submariners in the 1960s recorded strings of quick heartbeatlike pulses and nicknamed the unknown source a “bio-duck.” Whatever it is sounds off mostly in winter and spring in the Weddell Sea off Antarctica and the waters off Western Australia.

The sound is “way too loud for a fish,” says marine biologist Denise Risch of Integrated Statistics in Falmouth, Mass. Listeners have proposed sources from ...